Conventionally, a known headup display device is configured to control an indication brightness of an image according to a present illumination in an environment of a vehicle. Such a headup display device (HUD device) is further configured to project the image to a front windshield of the vehicle to indicate the image as a virtual image in an interior of the vehicle. For example, JP-A-2007-94130 proposes such an HUD device. The HUD device of JP-A-2007-94130 employs higher one of illumination in a front region of a vehicle and illumination in an upper region of the vehicle as a present illumination and indicates an image with a brightness according to the present illumination. The dashed dotted line of FIG. 8 shows an example of a correlation between the illumination in the front region of the vehicle and the illumination in the upper region of the vehicle. As shown by the region surrounded by the dashed line in FIG. 8, the relation between the illumination in the front region of the vehicle and the illumination in the upper region of the vehicle may collapse in a specific vehicle environment. The JP-A-2007-94130 employs the above indication operation in consideration of such collapse in the correlation in such a specific vehicle environment. The HUD device of JP-A-2007-94130 is configured to indicate the image with high brightness correspondingly to the illumination in the lighter upper region in a state where the upper region of the vehicle is light and the front region of the vehicle is dark. Therefore, the HUD device enables an occupant in an interior of the vehicle to recognize the image easily. The HUD device is also configured to indicate the image with high brightness correspondingly to the illumination in the lighter front region in a state where the upper region of the vehicle is dark and the front region of the vehicle is light. Therefore, also in this case, the HUD device enables an occupant in the interior of the vehicle to recognize the image easily.
It is noted that when the upper region of the vehicle is light and when the front region of the vehicle is dark, the interior of the vehicle becomes lighter than the exterior of the vehicle, in general. Therefore, in this case, eyes of an occupant in the interior of the vehicle are in a state of light adaptation (light-adapted state). The HUD device of JP-A-2007-94130 is configured to employ an indication brightness simply corresponding to the illumination in the lighter upper region. In this case, the brightness of the image may be insufficient for eyes of the occupant being in the light-adapted state. Consequently, the occupant may cause blurred visual recognition of the image. Alternatively, when the upper region of the vehicle is dark and when the front region of the vehicle is light, the interior of the vehicle becomes darker than the exterior of the vehicle, in general. Therefore, in this case, eyes of an occupant in the interior of the vehicle are in a state of dark adaptation (dark-adapted state). The HUD device of JP-A-2007-94130 is configured to employ an indication brightness simply corresponding to the illumination in the lighter front region. In this case, the brightness of the image may be excessive for eyes of the occupant being in the dark-adapted state. Consequently, the occupant may cause flickering visual recognition of the image.